diff --git a/sphinx/builders/__init__.py b/sphinx/builders/__init__.py
index 70ec6cb56..74cd6198b 100644
--- a/sphinx/builders/__init__.py
+++ b/sphinx/builders/__init__.py
@@ -247,8 +247,8 @@ class Builder(object):
         # while reading, collect all warnings from docutils
         warnings = []
         self.env.set_warnfunc(lambda *args: warnings.append(args))
-        updated_docnames = self.env.update(self.config, self.srcdir,
-                                           self.doctreedir, self.app)
+        updated_docnames = set(self.env.update(self.config, self.srcdir,
+                                               self.doctreedir, self.app))
         self.env.set_warnfunc(self.warn)
         for warning in warnings:
             self.warn(*warning)
diff --git a/sphinx/environment.py b/sphinx/environment.py
index 423273541..c3d67d733 100644
--- a/sphinx/environment.py
+++ b/sphinx/environment.py
@@ -590,7 +590,7 @@ class BuildEnvironment:
 
         self.app = None
         app.emit('env-updated', self)
-        return set(docnames)
+        return docnames
 
     def _read_serial(self, docnames, app):
         for docname in app.status_iterator(docnames, 'reading sources... ',